During routine locomotive maintenance, it is often required to move (or spot) a locomotive within or outside of a railway maintenance facility or shop. In many cases, it is desirable to do this without operating the diesel engine or other power source usually used for driving the locomotive and to move the locomotive in another manner.
Spotter control systems are in use that are operated from the locomotive cab and drive the locomotive by applying voltage from the locomotive batteries across two of the locomotive traction motors to develop power. Braking is achieved by operation of the locomotive cab brake controls after the brake system has been charged with air pressure.
For various reasons, it may be more convenient and efficient to have the spotting operation controlled from a point outside the locomotive where, for example, a single person my be able to safely and efficiently move the locomotive to a new position.